Introduction

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# Introduction - PowerPoint PPT Presentation

Introduction. Conventional encryption, also referred to symmetric encryption, secret key, or single key encryption. An encryption scheme has five ingredients: Plaintext: this is the original message Encryption Algorithm: performs various transformation and substitution on the plaintext.

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Presentation Transcript
Introduction
• Conventional encryption, also referred to symmetric encryption, secret key, or single key encryption.
• An encryption scheme has five ingredients:
• Plaintext: this is the original message
• Encryption Algorithm: performs various transformation and substitution on the plaintext
Continue…
• Secret Key: The exact substitution and transformations performs by the algorithm depends on the Key
• Cipher Text: this is the scrambled message produced as output.
• Decryption Algorithm: this is the encryption algorithm run in reverse
Essential Elements of a Conventional Encryption
• Source for a message which produces a message in plaintext.

M elements could be a 26 capital letters or nowadays the binary alphabet {0,1}

• A key of the form:

is generated at the source and is delivered to the

destination by the mean of secure channel.

Continue …
• The cipher text generated by this equation:
• The intended receiver, in possession of the key is able to invert the transformation by this equation:
Cryptography

Generally classified along three independent Dimensions:

• The type of operations used for transforming plain text to cipher text (substitution, and transposition {permutation, and matrix})
• The number of keys used
• Symmetric (single key)
• Public key or asymmetric (two keys)
• The way in which the plaintext is processed
• Block (cipher one block at a time)
• Stream (cipher the input continuously)
Cryptanalysis
• The process of attempting to discover the plain text or key is known as cryptanalysis.
• Brute–Force approach: knowing the algorithm and cipher text and trying all possible keys.
• Chosen Plaintext: if the opponent knows the placement of certain key words in the header of file
Average Time Required for Key Exchange
• An encryption scheme is computationally secure if :
• The cost of breaking the cipher text exceeds the value of the encrypted information.
• The time required to break the cipher exceeds the life time of the information.
Classical Encryption Techniques

One useful classification is:

• Substitution Technique: the letters of plaintext are replaced by other letters or by numbers, examples are (Caesar cipher and Mono-alphabetic ciphers, Hill cipher and Polyalphabetic cipher).
• Transposition techniques: performing some permutation on the plaintext letters such as Rail Fence algorithm and rotor machines.

(go to the net and find information about them)

Caesar Cipher Technique
• Each letter of the alphabet is replaced by the letter stands three places further down the alphabet:
• Note that the alphabet is wrapped around, so that the letter following Z is A
Continue…
• Then have Caesar cipher as:
• C = E(p) = (p + k) mod (26)
• p = D(C) = (C – k) mod (26)
• The important characteristics of this technique:
• The encryption and decryption algorithms are known.
• There are only 25 key to try which is far from security.
• The language of the plaintext is known